PHYSICS. H3 



without altering the direction of the line of vision. This is 

 effected by interposing between the object and the objective 

 a glass tank with plane parallel top and bottom, in which 

 some liquid is placed. The height of the surface is varied 

 by means of a suitable plunger. 



Gariel lias modified the ordinary projection instrument 

 known as the phenakisticope, removing the disk which car- 

 ries the figures, and doubling the number of lenses in order 

 to give more light on the screen. Placing now a fixed ob- 

 ject in the apparatus, as a round opening in a screen, for ex- 

 ample, its image may be made to persist by sufficiently rapid 

 rotation. Using two openings of different colors, their re- 

 sultant may be shown. With a prism, the spectrum given 

 when the apparatus is at rest becomes white when in mo- 

 tion. The apparatus shows the manometric flames very 

 well. 



Terquem has given the composition of a varnish which 

 may be spread on glass without injuring its transparency, 

 but which will enable it to take ordinary drawing-ink, so 

 that various devices for the lantern may be drawn upon it. 

 The varnish in question is composed of alcohol, 100 cubic 

 centimeters ; mastic, 7 grams ; sandarac, 3 grams. It has 

 been the practice for some years in this country to prepare 

 plates for receiving India-ink by flowing them with a dilute 

 solution of gelatin, one ounce to the pint of water. Very 

 fine drawings can be made on it, and it is easily removed by 

 water. 



3. Dispersion. 



Becquercl has examined carefully the ultra-red portions 

 of the spectrum, using the principle of phosphorescence for 

 the observation of this region. 



Campbell has devised a double slit which he has used 

 with good results for measuring the distances between the 

 lines in the spectrum, and which he finds of great service in 

 cases where the illumination is so slight as to prevent the 

 use of the micrometer. One slit is above the other, the up- 

 per one being movable at right angles to its length by a 

 micrometer screw of 200 threads to the inch, the graduated 

 head of which is capable of indicating one five-millionth of 

 an inch in the motion of the slit. If now a reading of the 

 micrometer be taken when the slits are superposed and form 



